1. Field of the Invention
The invention is generally directed to a device for the selective oxidation of a substance stream, such as a reformate stream, using a selectively active catalyst and an oxidation catalyst, and methods related to the same.
2. Description of the Related Art
EP 0 941 663 A1 describes a selective oxidizer, which is equipped—for the purpose of accelerating the cold-start process and shortening the cold-start time—with a combination of pellets coated with a selectively active catalyst, and with pellets that are coated with catalysts that possess oxidation activity at room temperature. In accordance with various embodiments in the above-mentioned EP publication, the pellets are distributed throughout the entire catalyst chamber, or are placed in the inlet region of chamber, so that during a cold-start of the selective oxidizer various substances will be oxidized at this catalyst and will heat the selective oxidation catalyst with the released thermal energy. This shortens the cold-start time.
EP 0 955 351 A1 describes a design in which seven stages are used to switch from a 100% oxidation catalyst active at room temperature in the inlet area to a 100% selective oxidation catalyst in the outlet area.
The designs set forth in the above-identified EP publications fulfill the well-known requirements of shortening the cold-start time, but they can lead to serious problems during regular operation. For example, since the oxidation catalyst is always present, it generates a large amount of heat during regular operation, which is a disadvantage for the selective oxidation process, since the selective oxidation requires a comparatively narrow temperature range between approximately 200° C. and 300° C. to take place with optimum efficiency. Consequently, during regular operation, a configuration that includes oxidation catalyst requires appropriate cooling or increased cooling at the oxidation catalyst.
A further disadvantage is that the oxidation catalyst not only converts the component of the substance stream that is supposed to be selectively oxidized (e.g., CO), but also other components present in the stream. In applications associated with hydrogen gas generation for fuel cells, it is possible for hydrogen to be oxidized at the oxidation catalyst, which results in some of the hydrogen no longer being available for the fuel cell, and can result in hydrogen inhibiting the desired selective oxidation by excessively heating the selective oxidation catalyst due to its exothermic reaction at the oxidation catalyst. Both of these disadvantages have detrimental effects on the efficiency of the overall system, on the achieved purification quality, and on the degree of carbon monoxide concentration reduction in the reformate gas stream or any other comparable substance stream.
Consequently, there remains a need for a device that can selectively oxidize a substance stream, particularly one that can combine desirable cold-starting characteristics with desirable energy-saving characteristics during regular operation. The present invention fulfills these and other needs, and provides further related advantages.